Copier with leading edge image control

ABSTRACT

An image producing process of a document copier is controlled by means detecting a low average intensity of reflected light during initial stages of scan of an original being copied to produce a low density or light copy adjacent the leading edge of the copy sheet passing through the machine thereby minimizing paper feed difficulties associated with the passage of copy having a dense leading edge through a contact fuser.

United States Patent [191 [111 3,834,807 Fuller et al. Sept. 10, 1974 COPIER WITH LEADING EDGE IMAGE [56] References Cited CONTROL UNITED STATES PATENTS [75] Inventors: Sterritt Ray Fuller; Howard Everett 3,135,179 6/1964 Cerasani et al. 355/8 Munzel; Carl Allen Queener, all of Lexington, Ky. Primary ExaminerRobert P. Greiner [73] Assignee: International Business Machines Attorney Agent or Flrm E' Ronald Coffman Corporation, Armonk, N.Y. ABSTRACT [22] Filed: 1974 An image producing process of a document copier is [21] Appl. No.: 442,533 controlled by means detecting a low average intensity of reflected light during initial stages of scan of an original being copied to produce a low density or light [52 US. Cl. 355/3 R, 355/8, 1 gag/.751, copy adjacent the leading edge Ofthe copy Sheet paw 51] I t Cl G03 15/00 ing through the machine thereby minimizing paper [58] g llg7/l7 5. feed difficulties associated with the passage of copy having a dense leading edge through a contact fuser.

4 Claims, 4 Drawing Figures PATENTEDSEPI 01 M 3,834.80?

65 IMAGE AREA V TRAILING EDGE 2 e2 IMAGE AREA 9 LEADING EDGE ERASE LAMP DELAY SWITCH FIG. 4

+ our 1 COPIERWITH LEADING EDGE IMAGE CONTROL BACKGROUND OF THE INVENTION Contact fusers are known to be efficient, compact and versatile means of fixing thermoplastic powder images to support sheets. By theme of good design techniques giving particular attention to heat transfer and material selection, a contact fuser can be constructed to fuse the thermoplastic image and press it against a support sheet without becoming adhesively attached to it. Adhesion of toner to the fuser contact surface can soil subsequent copy and in severe cases can stick the copy sheet to the fuser surface causing a paper jam. We have found that a maximum tendency for adhesion between a copy being fused and a heated fuser roll of a contact fuser exists when a broad area of a very dense image is being fused. The adhesive tendency problem is greatest where the broad dense image is adjacent the copy sheet leading edge where separation of the copy sheet from the heated fuser roll must be initiated.

In the copying of ordinary documents such as letters, line drawings, and even photographs, it is not usual to find large solid black image areas at one edge of the original. Where a copy image is produced by exposure to light reflected from the original document, the absence of a document to reflect light will result in creation of a dense black image over the whole area outside the document boundary. For this reason, reflected light copy machines are generally provided with a document cover that presents a white background that is superimposed over those documents that are smaller than the exposure window of the copy machine. A su perimposed white backing is not effective, however, when copying from a page of a thick document such as a book or in any other case where the operator fails to close the cover. Books whose pages are substantially smaller than the exposure window of a copy machine thus create the propensity for producing copy having one or more solid black edges. If one of these edges is the leading edge of a copy sheet as it passes through the copy process, a maximum possibility exists for a paper feed failure in a contact fuser.

DISCLOSURE OF THE INVENTION We have devised an apparatus which operates during a progressive exposure window scan for sensing the average reflected light intensity from the exposure window. Normally operative inter-image erase circuitry activates an erase lamp at the beginning of a copy cycle to prevent development of a photoconductor in fixed regions outside the pre-set image area. During exposure scan, control circuitry compares the sensed average light intensity with a pre-set threshold value. When the reflected light sensed exceeds the threshold, the erase lamp is extinguished and the copy process proceeds normally. The copy produced will have an unobjectionable white edge which readily passes through the contact fuser. Additionally, toner usage and cleaning requirements are reduced.

Our invention can be implemented by a variety of techniques that vary from the preferred embodiment as described generally above. The image on the leading edge need not be entirely erased to obtain the benefits of our invention. Thus, by increasing the electrical bias voltage on a magnetic brush in the developer station, a low density image can be produced in the region of the leading edge which would preserve any information that might be found in an original having a dense image in the vicinity of the leading edge. The leading edge image can also be lightened or eliminated by control of the transfer station or, if timing considerations permit, control of the sensitizing charge corona. While we prefer to sense average light intensity by a photodetecting device mounted on a member that remains positionally associated with the scan footprint of reflected light such as a scanning lens, other arrangements for average light determination can be employed. It is important, however, that only light reflected from at least a-repre sentative sample of a small transverse scanning section across the document window be permitted to reach the light detector in order to assure a precise time relation between the threshold detection and the copy process that is controlled. It is possible, although more expensive and less reliable, to detect average light intensity through the use of multiple light detectors located closely adajcent the exposure window itself and providing averaging circuitry for electrically combining the output of all detectors before threshold comparison.

As mentioned above, copy machines ordinarily are provided with a document cover having a light background that normally obviates the problem to which our invention is directed. For copy machines capable of making copies from thick documents such as books, provision is made for the document cover to be raised to accommodate the book thickness. We prefer to include in the implementation of our invention, means sensitive to the position of the document cover for enabling the leading edge erase control only when document cover is in its raised position. Thin documents having dark images adjacent their edges thus can be copied in their entirety by a machine equipped with our invention, whereas automatic leading edge-detection and density control will be performed whenever copy is being made with the document cover open.

These and other objects, features and advantages of our invention will be apparent to those skilled in the art from the following more specific description of an illustrative preferred embodiment wherein reference is made to the accompanying drawings of which:

FIG. 1 shows in simplified form, a xerographic type copy machine or printer employing leading edge density control in accordance with our invention;

FIG. 2 is a logic diagram of the control for our invention;

FIG. 3 is a perspective exploded view of a light sensing device employed in our invention; and

FIG. 4 is a circuit diagram of an approriate threshold detecting circuit employed in the control logic of FIG. 2.

More specifically, in FIG. 1 there is shown a xerographic type copy machine or printer 10 including a rotatable drum or image production member 11 having a photoconductive peripheral surface 12 that is movable along a copy production path past electrostatic imaging operation performed by a sensitizing charge corona 13, an image exposing station 14; a magnetic brush developing station 40 at which a toner powder image 15 is developed on surface 12, and a transfer corona station 16 at which the powder image 15 is transferred to a copy sheet 17. A cleaning station 18 removes any residual toner particles from. the photoconductive surface 12 prior to the surface being represented to the sensitizing charge corona 13.

At exposing station 14, optical scanning apparatus 20 within a light tight housing 30 causes a flowing segmental light image reflected from a document 19 supported on exposure window 31 to be presented to the photoconductive surface 12 in synchronism with its movement along path 12a. This flowing image is created by the movement of an optical element such as a lens 21 along a scanning path 21a that is parallel to one axis of the exposure window 31. Lens 21 is mounted on a scan producing carriage 22 which in turn is supported by suitable rails or tracks 23. Carriage 22 is driven at an appropriate speed to provide the flow image synchonous with drum 11 through a transmission 24. A flexible diaphragm or curtain member 32 surrounds the lens 21 and assures that only light passing through lens 21 can reach photoconductive surface 12. A pair of normally stationary mirrors 33 and 34 re-direct the light received from lens 21 to the photoconductive surface 12.

To maximize light efficiency and to minimize the effects of the curvature of photoconductive surface 12, only a very narrow segment or footprint 25 of the exposure window 31 is projected through lens 21 at any given time. This is accomplished by a stationary incandescent lamp 35 which directs a narrow transverse beam of light through stationary mirror 36 and moving mirror 26 onto exposure window 31 to illuminate only the narrow footprint 25. Moving mirror 26 is supported by a carriage 27 and suitable rails or tracks 28 for movement by transmission 24 at a speed suitable to progressively scan the illumination footprint 25 through successive segments or image areas of the window 31 in optical synchronism with the movement of lens 21 and photoconductive surface 12. The optical devices and scanning system thus described are more fully described in U.S. Pat. application Ser. No. 420,338 entitled OPTICAL SCANNING SYSTEM, filed by R. T. Ritchie and J. C. Rogers on Nov. 27, 1973, and in U.S. Pat. No. 3,758,774,.

Magnetic brush developing station 40 presents electroscopic toner powder to a latent electrostatic charge image created by exposing station 14. Developing station 40 includes a magnetic brush unit 42 having an electrically biased conductive cylinder 44 which performs the functions of a development electrode and thus is capable of faithfully reproducing half-tone and broad area solid black images as well as line images.

The powder image generated by developing station 40 is transferred to the copy sheets 17 which are presented in synchronism with the movement and position of photoconductive surface 12. Copy sheets 17 are transported from the transfer station 16 by a pneumatic conveyor 50 through an image fixing contact fuser 51 where the powder image is permanently bonded to the copy sheet 17. Exit feed rolls 52 deliver the finished copy sheet to an exit pocket 53 for removal. Contact fuser 51 can be like that described in U.S. Pat. application Ser. No. 317,482 of James A. Machmer entitled HIGH SPEED PRINTING SYSTEM WITH HEATED ROLL FUSER (LE9-72-0l9). The fuser consists of a heated fusing roll 54 having an internal heat source and a heated surface 55 formed of a deformable silicone elastomer having an inherent low adhesive quality with respect to the thermoplastic toner material of powder image 15. The fuser 51 further comprises an unheated backup or pressure roll 56 which brings the copy sheet 17 and the powder image thereon into intimate contact with the heated surface 55.

The problem to which our invention is addressed occurs principally when a document to be copied is placed on exposure window 31 in such away as to leave an uncovered portion 29 adjacent the edge of window 31 that is initially scanned by lens 21. A'document hold down cover 37 is pivotally mounted adjacent exposure window 31 and carries a white background surface member 38 which can be placed overa thin document positioned on exposure window 31 to cover regions such as 29. Thus, the uncovered margin portion 29 of the exposure window 31 will occur most commonly when undersized documents such as the page of a small book are being copied and the thickness of the document prevents closing of cover 37. With no document or white background member to reflect light from mirror 26, lens 21 will project little light to photoconductive surface 12 and the still highly charged photoconductive surface 12 will be presented to the developing station 40 where it will be developed as a large solid black image. Without our invention, this black image would be transferred to the copy sheet 17 in registration with its leading edge. The presence of the large black image across the leading edge of a copy sheet 17 has demonstrated a propensity to remain adhered to the fuser roll surface 55. Failure of copy to properly separate from fuser roll surface causes a paper jam requiring shut-down of the entire machine 10. Large dense images on a copy sheet 17 at locations other than the leading edge are of less consequence. For example, once a properly separated leading edge has entered exit feed rolls 52, the copy sheet 17 can be pulled from the fuser roll surface 55 should a tendency toward adhesion develop.

It has been previously known to provide a light source for discharging regions on a photoconductive surface between successive image areas to minimize toner consumption and to extend the life of a filter unit that might be part of a cleaning station 18,. In our invention, an elongated discharge or erase lamp 60 is positioned between exposing station 14 and developing station 40. This discharge lamp 60 normally serves to provide the known function of erasing or discharging the photoconductor 12 in the region following the trailing edge of a first copy and preceeding the leading edge of the image area for a successive copy. Inasmuch as leading and trailing edges of copies are always presented to the same locations on the surface 12 of drum 11, control of discharge lamp 60 can be conveniently provided by a drum position sensing device such as cam 61 and associated switches 62 and 63. Cam 61 is configured toclose switch 62 at the time that the leading edge of the image area on photoconductive surface 12 arrives in alignment with discharge lamp 60. Switch 63 is closed at the time'that the trailing edge of the image area on photoconductive surface 12 passes the discharge lamp 60. As shown in FIG. 2, a latching switch 64 responds to closure of switch 63 to make discharge lamp 60 operative. When switch 65, hereinafter described, senses that cover 37 is down by being positioned in its broken line position, latching switch 64 responds to closure of image area leading edge indicative switch 62 to extinguish discharge lamp 60, making it unoperative.

Our invention controls the density of the leading edge of copy produced at transfer station 16 by providing an alternate contol for turning off lamp 60. Preferably, our alternate control is selected by cover down sensing switch 65 being positioned in its full line position in FIG. 2 as an indication that document cover 37 is in an open position. Our control employs a reflected light intensity sensing device 70 including a circuit 80 for detecting the rise of reflected light intensity above a preset threshold for initiating a signal to reset latching switch 64 and make discharge lamp 60 unoperative after a time measured by a delay device or circuit 66 that is a measure of the time required for a point on photoconductive surface 12 at exposure station 14 to move into alignment with discharge lamp 60. Thus, with cover 37 in its up or open position and switch 65, thereby being in its full line position as shown in FIG. 2, lamp 60 will remain ON beyond the leading edge of the image area on photoconductive surface 12 normally defined by the closure of switch 62.

Light intensity sensing device 70 will sense a low level of light so long as mirror 26 projects light through window 31 in regions where the light is not reflected back by a document 19 thereon. As the scanning band of light from mirror 26 reaches the left or leading edge of document 19, the light intensity received by sensing device 70 will rise rapidly and a signal will be initiated to delay circuit 66.

Lamp 60 will remain ON to discharge photoconductive surface 12 until the delay period has expired, at which time the electrostatic image of the left edge of document 19 will reach the location of discharge lamp 60. Completion of the delay time will transmit the turnoff signal to latching switch 64 which will extinguish the lamp 60 and allow the copy process to proceed in a normal manner. Since the leading edge of the'image area on photoconductive surface 12 has been discharged by lamp 60, no toner material will be developed onto the photoconductive surface 12 by developing station 40 and no image will be transferred to the leading edge of copy sheet 17 at transfer station 16. The untoned leading edge of the copy will pass readily through the contact fuser 51.

By providing switch 65, it is possible to copy the full area of thin documents that may include dark portions adjacent the leading'edge scan region of exposure window 31.

The light intensity sensing device 70 is better shown in FIG. 3. This device comprises a photo transistor 90 that is mounted under a light collecting condensing lens 71 on a printed circuit board 72 within a housing 73. The housing 73 includes baffle walls 74 and 75 that limit the access of light to within the housing 73 to prevent direct impingement of light from lamp 35 (FIG. 1) onto phototransistor 90. Tapered baffel walls 75 limit the lateral extent of the field of view of phototransistor 90. The field of view should be large enough to obtain a usefully representative sample of the light reflected from the exposure window 31 to distinguish between the presence and absence of a document thereon. Due to the geometry of the scanning system shown, an opening 76 is provided in right side wall 77 of the housing 73 to enable phototransistor 90 to receive reflected light throughout the entire scan movement of lens 21 and mirror 26. The condensing lens 71 is mounted directly above the phototransistor 90 to assist in collecting light from a broad area of the exposure window 5.7. The collected light from a broad area thus represents an average intensity of the narrow illumination footprint 25 which is particularly suited for distinguishing between the presence and absence of a document on exposure window 57.

As is known in the art, an incandescent lamp such as 35 generates a substantial amount of infrared light along with its visible light. We prefer to employ a dichroic cold mirror 36 that reflects only the visible light onto exposure window 31 thereby minimizing the tendency to heat the document 1.9 on the exposure window 31. The infrared light simply passes through the dichroic mirror 36 and is reflected around within the optical system housing 30. The inner walls 39 of housing 30 are colored with a flat black material to minimize internal reflections, however, the large amount of infrared produced by lamp 35 is not completely absorbed. To minimize undesired optical noise effects produced by such infrared light, we have provided a blue-tint filter 78 that is superimposed over lens 71 and thus prevents transmission of infrared light to the phototransistor 90.

The threshold detecting circuit 80 carried by printed circuit board 72 within the housing 73 is shown in FIG. 4. The detecting circuit 80 is provided with a positive power supply input 81, a negative power supply input 82, and a ground input 83. The input positive power is smoothed by resistor 84 and capacitor 85 and is applied to similar circuit branches 91 and 92 to derive comparative voltages on lines 93 and 94 that are input to an operational amplifier 86 connected by resistor 87 as a Schmitt trigger to generate a precise fast output on line 88. A circuit branch 91 includes phototransistor 90 whose operating status is altered in proportion to the intensity of light applied thereto, voltage dividing resistors 91a and 91b, base resistor and load resistor 96. Resistor 91b includes a variable potentiometer tap 97 by which the threshold to which the circuit 80 responds can be adjusted. A voltage level thus is derived on line 93 that is a function of the light level intensity applied to phototransistor 90 and a particular setting of threshold adjustment potentiometer tap 97. Branch 92 includes voltage dividing resistors 92a and 92b and load resistor 98 in addition to a diode 97 which provides a forward voltage drop that varies with ambient temperature in a manner that is similar to the temperature variation voltage drop across phototransistor 90 thereby negating effects of ambient temperature on the signals presented to lines 93 and 94.

In operation, output line 88 normally presents a positive potential so long as the light received by phototransistor 90 is below the threshold set by potentiometer tap 97. As the light level rises to and beyond the threshold level, operational amplifier 86 changes the polarity of output line 88 to negative, thereby detecting the occurrence of the threshold crossing. Potentiometer tap 97 is made adjustable to allow tuning of the particular circuit components involved and also provides for adjustment of the threshold for a particular machine upon its installation to take into account the general room light level within its immediate environment.

Delay device 66 can be a simple single shot circuit whose time constant is set to correspond to the time required for a point on photoconductive surface 12 to pass from exposure station 14 to lamp 60. For a machine having predictable constant velocity such a device would work well. Maximum precision can be obtained by providing a displacement emitter operating in synchronism with the drum 11 to provide a delay that is measured directly by the movement of the photoconductive l2.

While we have described a specific preferred embodiment of our invention wherein a light sensitive phototransistor is carried by a scanning lens and controls an erase lamp to preventdevelopment of a dense leading edge, those skilled in the art will recognize various modifications, additions and deletions can be made to our embodiment without departing from the inventive concepts we have taught. For example, instead of employing an erase lamp, it is possible to reduce the density of the leading edge of copy developed by increasing the bias voltage on the magnetic brush roll 42 to a level approaching the charge level on the photoconductive surface 12. Also, it is possible to selectively control the transfer corona 16 so as to not transfer the heavily toned leading edge portion of a copy produced from an undersized document. Furthermore, the photosensitive device can be located other than on a moving optical element. Even a stationary photosensing device will work satisfactorily if it is located to substantially uniformly receive a representative sample of the reflected light from the illuminated increment of the exposure window 57'. Accordingly, the subject matter patented is to be limited only by the appended claims.

We claim:

1. A printer having an image productionmember including a photoconductive surface that is movable along a copy production path, electrostatic imaging, powder image developing, and powder image transfer stations positioned sequentially along said copy production path, said imaging station comprising means for progressively scanning small increments of a light image that are reflected from material positioned on an exposure window onto successive portions of said photoconductive surface, said developing station comprising electrode means capable of developing dense solid black images, said copier further comprising image fixing means for receiving copy sheets to which powder images have been transferred at said transfer station and including a heated surface for contact fusing said powder images to said copy sheets, wherein the improvement comprises: 1

means having operative and unoperative states for altering the density of powder images transferred to said copy sheets to produce when operative, a lower than otherwise image density thereon,

means operative during said progressive scanning for sensing the instantaneous average intensity of a representative sample of said reflected light image increment, and

means responsive to detection of a rise in said instantaneous average reflected light intensity above a predetermined threshold valve for controlling said density altering means to its unoperative state.

2. A printer as defined in claim 1 wherein said exposure window is stationary and said progressive scanning means comprises a scanning member carrying an optical element through a scanning path extending along one axis of the exposure window and where the improvement further comprises:

said reflected light sensing means including-a light intensity detector supported on said scanning member.

3. A printer as defined in claim 1 further comprising a document hold down cover including a white background piece that is selectively positionable in close adjacency with said exposure window or alternately displaced therefrom, and wherein the improvement further comprises:

means responsive to said cover being in its position of close adjacency with said exposure window for disabling said density altering means.

4. A printer as defined in claim 1 wherein density altering means comprises an erase lamp positioned along said copy production path between said charging and developing stations, and wherein said light value rise detection responsive means comprises delay means for extinguishing said erase lamp upon the elapse of a period of time following said detection that is a measure of the time required for a point on said photoconductive surface to move from said exposing station to said developing station. 

1. A printer having an image production member including a photoconductive surface that is movable along a copy production path, electrostatic imaging, powder image developing, and powder image transfer stations positioned sequentially along said copy production path, said imaging station comprising means for progressively scanning small increments of a light image that are reflected from material positioned on an exposure window onto successive portions of said photoconductive surface, said developing station comprising electrode means capable of developing dense solid black images, said copier further comprising image fixing means for receiving copy sheets to which powder images have been transferred at said transfer station and including a heated surface for contact fusing said powder images to said copy sheets, wherein the improvement comprises: means having operative and unoperative States for altering the density of powder images transferred to said copy sheets to produce when operative, a lower than otherwise image density thereon, means operative during said progressive scanning for sensing the instantaneous average intensity of a representative sample of said reflected light image increment, and means responsive to detection of a rise in said instantaneous average reflected light intensity above a predetermined threshold valve for controlling said density altering means to its unoperative state.
 2. A printer as defined in claim 1 wherein said exposure window is stationary and said progressive scanning means comprises a scanning member carrying an optical element through a scanning path extending along one axis of the exposure window and where the improvement further comprises: said reflected light sensing means including a light intensity detector supported on said scanning member.
 3. A printer as defined in claim 1 further comprising a document hold down cover including a white background piece that is selectively positionable in close adjacency with said exposure window or alternately displaced therefrom, and wherein the improvement further comprises: means responsive to said cover being in its position of close adjacency with said exposure window for disabling said density altering means.
 4. A printer as defined in claim 1 wherein density altering means comprises an erase lamp positioned along said copy production path between said charging and developing stations, and wherein said light value rise detection responsive means comprises delay means for extinguishing said erase lamp upon the elapse of a period of time following said detection that is a measure of the time required for a point on said photoconductive surface to move from said exposing station to said developing station. 